The current standard insulating materials in liquid filled transformers are cellulosic materials of various thicknesses and density. Cellulose-based insulating materials, commonly called Kraft papers, have been widely used in oil-filled electrical distribution equipment since the early 1900's. Despite some of the shortcomings of cellulose, Kraft paper continues to be the insulation of choice in virtually all oil-filled transformers because of its low cost and reasonably good performance. Unfortunately, the cellulose polymer is subject to thermal degradation and vulnerable to oxidative and hydrolytic attack.
In general, cellulose-based insulating materials are used to insulate five different parts of the internal structure of the transformer. They consist mainly of: (1) turn-to-turn insulation of magnet wires; (2) layer-to-layer insulation; (3) low-voltage coil-to-ground insulation; (4) high-voltage coil-to-low voltage coil insulation; and (5) high-voltage coil-to-ground insulation.
The low-voltage coil-to-ground and the high-to-low voltage coil insulations usually consist of solid tubes combined with liquid filled spaces. The purpose of these spaces is to remove the heat from the core and coil structure through convection of the medium, and also help to improve the insulation strengths. The internal turn insulation is generally placed directly on the rectangular magnet wires and wrapped as paper tape. The material that is chosen to insulate the layer-to-layer, coil-to-coil and coil-to-ground insulation is according to the insulating requirements. These materials may vary from Kraft paper that is used in smaller transformers, whereas relatively thick spacers made of heavy cellulose press board, cellulose paper or porcelain are used for higher rating transformers.
The following are areas of importance describing the current art.